Hi,

I am working on a 3.7V battery system. For my speaker I use an amplifier running on 5V generated by a boost regulator (L6920). The ripple from the boost regulator is pretty high and gives anoying noise on the speaker. Especially when the load on the boost regulator is low the noise is annoying. I see that ripple is a problem with other boost regulators as well.

Anyone have a suggestion for a different boost regulator or a way to get rid of the noise on the speaker?

 

There are a least two ways to minimize the noise generated.

One is you may need more filtering on both the input of the boost regulator to minimize high frequency ground noise, and on output to minimize ripple.

The other is you need a good ground from the regulator back to your battery. This ground should not go through the amplifier ground.

How is the regulator constructed?

 

On the regulator I have 3 capacitors for filtering on the input: 47uF, 100nF and 1nF. On the regulator output I also have 3 capacitors: 47uF, 100nF and 1nF.

So my amplifier should have it's own ground separated from my power circuits ground. Since I have a 4 layer board I now use an internal layer for all grounds.

The regulator is constructed like this (taken from the datasheet):
"In L6920 the control is based on a comparator that continuously checks the status of output voltage. If the output voltage is lower than the expected value, the control function of the L6920 directs the energy stored in the inductor to be transferred to the load."

 

Using a common ground for digital and analog circuits generally causes noise from the digital getting into the analog. If you already have a board made then it could be difficult to solve the problem. Try adding some larger caps to see if that helps.

 

With those caps, it should be giving enough PSU noise reduction. The noise is probably coming into the front end of the amplifier.

What type and gain is the amp? What is the signal? How have you shielded the input wires and wires to the volume pot etc?

 

I will make a new revision of the PCB and are open for suggestions for this revision. If I want to separate analog and digital ground, should I put the digital ground on a different layer than my ground layer?

The gain on the amplifier (SSM2305) is 2. The signal is coming from the DAC on my MCU (STM32). The volume is controlled by the amplitude of the signal from the DAC.

 

If you are stuck with a board layout and can only adjust component values, increase the inductor. You have to recalc everything to make sure you still get the output current range you want, but bigger L = lower Vpp. And look at the output electrolytic cap type. There are many different flavors of electrolytics, and some are grown specifically for switcher outputs.

http://cds.linear.com/docs/en/application-note/an19fc.pdf

This is the bible for boost, with chapters on the input and output caps, plus a 2nd stage LC filter.

ak

 

I will make a new revision of the PCB and are open for suggestions for this revision. If I want to separate analog and digital ground, should I put the digital ground on a different layer than my ground layer?
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You should have separate ground planes for the analog and digital portions of the circuit but it can be a split plane on the same layer. You don't want the two planes to overlap if possible. All the digital signals should run only over the digital plane and all the analog signals only over the analog plane.

Tie the two planes together at a single point, preferably where there may be circuits that have both analog and digital signals such as an A/D converter. It may help the noise if the two planes are tied together with a ferrite bead. If you aren't sure where the best point for the single point connection should be, make pads at several locations for the jumper and experimentally determine which is best after you fab the board.

 

Yes, I may have to use separate ground planes for the next revision. Thanks for every reply!